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Both versions of the FDC boards use the WD1791; the later board uses the WD1691 (20 pin DIP) and WD2143 (18 pin DIP) clock generator in addition, rather than a bunch of random logic. The two look like any other 20 pin/18 pin 0.300 DIP, so it's not obvious from a casual glance.

Do you have the TRS-80 Model II Technical Reference Manual? That and the Shugart SA800-801 OEM and Users Manual should have the info you need. They should be available online. If you have trouble finding them let me know.

I have all those but unfortunately, that doesn't mean I always understand what I'm looking at.

(Side note: I haven't been able to find the Model II - Tech Ref online anywhere - if it's there, please let me know as I was thinking of scanning the whole thing so it can be available to others, and I won't bother if it's already there).

It gets screwy (for me anyway) in that the drive in the Model II is a Shugart SA-800, and the drive in the Disk Expansion Unit is a Magnetic Peripherals/Control Data 9404-B (info for which is also incl in the Tech Ref).
I'm not sure whether I'm supposed to be using the terminating info for the Shugart or the Control Data drive. The Disk Expansion Unit section of the Tech Ref refers me to the Control Data schematics for the termination stuff.

If it's a case of taking a 50 pin connector and soldering some resistors between certain pins, I can do that - I just can't figure out which pins the restistors should go between.

Both versions of the FDC boards use the WD1791; the later board uses the WD1691 (20 pin DIP) and WD2143 (18 pin DIP) clock generator in addition, rather than a bunch of random logic. The two look like any other 20 pin/18 pin 0.300 DIP, so it's not obvious from a casual glance.

Basically it loops signal lines back to terminator resistors on the internal disk drive. Without this "terminator", the disk drive chain would not be properly terminated.

Odd numbered connections are grounded so you can use a 7 pin flat cable to wire the loopback to a 50 pin connector. (see attached .jpg photo)

This loopback connector works fine on the one of my four model II's that uses the first generation floppy controller.

Kevin

Originally Posted by Lorne

I'm in need of a terminator pack for a Model II.
Its the thing that plugs into the back of the Model II where the Expansion Box cable would go, so that you can boot from the Model II without the Expansion Box.
It would be a brown & black plastic thing approximately 4" long, 3/4" high, and 1 1/2" deep.

Does anyone have one they don't need or would otherwise be willing to part with?

(Side note: I haven't been able to find the Model II - Tech Ref online anywhere - if it's there, please let me know as I was thinking of scanning the whole thing so it can be available to others, and I won't bother if it's already there).

There are actually 2 Technical Manuals. One from 1980 and another from 1981. Also 2 Service manuals. They are in a TRS-80 Archive. What is the best site to submit these files so the community will have the best access to them.

That terminator makes absolutely no sense. From the schematic, 4, 6, 8 are NC and 10 is the "2 sided" output from the drive. 40,38,36 and 34 are Write gate, Write Data, Step, and Direction.

If an internal drive has no termination packs, this will accomplish nothing. If the internal terminator is present, there's no need for this jumper setup.

Can anyone explain this one to me?

I had the same issue when I thought I needed the terminator. Having no info then, I concluded that it could only be some kind of loopback. But the schematics didn't provide any clue which pins should be connected. But I'm glad someone provided the info even if it makes no sense, other than as another Tandy money source.

What is the best site to submit these files so the community will have the best access to them.

Lorne has pointed out to me that the earlier termination scheme with the jumper block relies on a peculiar wirewrap patch to the SA800, wiring the "user I/O" pins to spare terminator header pins. In other words, a terrible kludge...